A plastic sintered body that has been conventionally used as a filter, silencing body, or the like is a porous body formed by packing plastic powder into the cavity of a metal mold, heating the metal mold at a temperature close to the melting point of the plastic powder, welding adjacent particles of the plastic powder, and cooling the metal mold. The plastic sintered body has contiguous voids through which gas or liquid passes.
FIGS. 4A-4C illustrate a conventional method for manufacturing a two-layer plastic sintered body 100. The two-layer plastic sintered body 100 illustrated in FIGS. 4A-4C is formed by the first and second sintering processes. In the first sintering process, as illustrated in FIG. 4A, the first plastic powder is packed in a cylindrical cavity 102 of a first metal mold 101 and an opening 103 of the cavity 102 is covered with a lid (not illustrated), the first metal mold 101 is put in a heating furnace heated to a temperature close to the melting point of the first plastic powder, and the entire first metal mold 101 is heated in the heating furnace for 20 to 30 minutes to sinter the first plastic powder in the cavity 102. As a result, a cylindrical first plastic sintered body 104 is formed. In the first plastic sintered body 104, interconnected minute voids are formed between particles of the first plastic powder.
In addition, in the second sintering process, as illustrated in FIG. 4B, the first plastic sintered body 104 is accommodated in a second metal mold 105, the second plastic powder having a particle size different from that of the first plastic powder is packed in a cylindrical cavity 106 of the second metal mold 105 formed on the radially outer side of the first plastic sintered body 104, an opening 107 of the cavity 106 is covered with a lid (not illustrated), the second metal mold 105 is put in the heating furnace heated to a temperature close to the melting point of the second plastic powder, the entire second metal mold 105 is heated in the heating furnace for 20 to 30 minutes to sinter the second plastic powder in the cavity 106. As a result, a cylindrical second plastic sintered body 108 is integrally formed without any space on the outer circumferential side of the first plastic sintered body 104, and the two-layer plastic sintered body 100 including the first plastic sintered body 104 and the second plastic sintered body 108 is formed (see FIG. 4C). In the two-layer plastic sintered body 100, the first plastic sintered body 104 includes voids having a size different from that of the second plastic sintered body 108 (see JP-A-2008-221611).
However, in the method for manufacturing the conventional two-layer plastic sintered body 100 illustrated in FIGS. 4A-4C, since a plurality of metal molds (the first metal mold 101 and the second metal mold 105) corresponding to the sintering processes are necessary and these metal molds need to be input to or output from the furnace for each of the plurality of sintering processes, the cost of metal molds becomes high and the number of manufacturing man-hours increases. In addition, in the method for manufacturing the conventional plastic sintered body 100 illustrated in FIGS. 4A-4C, since a plurality of types of plastic powder having different particle sizes need to be prepared and the plastic powder with a difference particle size needs to be selectively used for each of the sintering processes, the manufacturing work becomes complicated.
An object of the invention is to provide a method for manufacturing a plastic sintered body, metal mold, and plastic sintered body that can reduce the cost of the metal molds and the number of manufacturing man-hours and simplify the manufacturing work.